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<tr class="t-nv"><td colspan="5"> <a href="atomic.html" title="c/language/atomic">原子类型</a> <span class="t-mark-rev t-since-c11">(C11)</span></td></tr>
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<p>声明拥有以位数表示的显式宽度的成员。相邻的位域成员可能被打包，共享和分散到单独的字节。
</p><p>位域声明是使用下列<a href="declarations.html" title="c/language/declarations">声明器</a>的<a href="struct.html" title="c/language/struct">结构体</a>或<a href="union.html" title="c/language/union">联合体</a>成员声明：
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <span class="t-spar">标识符</span><span class="t-mark">(可选)</span> <code><b>:</b></code> <span class="t-spar">宽度</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">标识符</span>
</td>
<td> -
</td>
<td>  正在声明的位域名称。名称是可选的：无名位域引入一个指定的填充位数
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">宽度</span>
</td>
<td> -
</td>
<td>  一个拥有大于或等于零，且小于或等于底层类型位数的整数<a href="constant_expression.html" title="c/language/constant expression">常量表达式</a>。大于零时，此为此位域将占据的位数。零值仅允许用于无名位域，并拥有特殊含义：它指定结构体定义中的下个位域会从分配单元边界开始。
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>位域可以拥有四种类型之一（可为 <a href="const.html" title="c/language/const">const</a> 或 <a href="volatile.html" title="c/language/volatile">volatile</a> 限定）：
</p>
<ul><li> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">int</span></span></span> ，对于无符号位域（ <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">int</span> b<span class="sy4">:</span><span class="nu0">3</span><span class="sy4">;</span></span></span> 拥有范围 <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">0</span>..<span class="nu0">7</span></span></span> ）
</li><li> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">int</span></span></span> ，对于有符号位域（ <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">int</span> b<span class="sy4">:</span><span class="nu0">3</span><span class="sy4">;</span></span></span> 拥有范围 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">-</span><span class="nu0">4</span>..<span class="nu0">3</span></span></span> ）
</li><li> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> ，对于拥有实现定义符号性的位域（注意这与其他所有地方都表示“ <code>signed int</code> ”的关键词 <code>int</code> 含义不同）。例如， <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span> b<span class="sy4">:</span><span class="nu0">3</span><span class="sy4">;</span></span></span> 可能拥有 <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">0</span>..<span class="nu0">7</span></span></span> 或 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">-</span><span class="nu0">4</span>..<span class="nu0">3</span></span></span> 范围的值。
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<ul><li> <span class="t-c"><span class="mw-geshi c source-c">_Bool</span></span> ，对于单个位的位域（ <span class="t-c"><span class="mw-geshi c source-c">bool x<span class="sy4">:</span><span class="nu0">1</span><span class="sy4">;</span></span></span> 拥有范围 <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">0</span>..<span class="nu0">1</span></span></span> ），来自它和到它的<a href="conversion.html" title="c/language/conversion">隐式转换</a>遵循布尔转换规则。
</li></ul>
</td>
<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td></tr>
</table>
<p>可接受额外的实现定义类型。<span class="t-rev-inl t-since-c11"><span>位域是否可以拥有<a href="atomic.html" title="c/language/atomic">原子</a>类型也是实现定义的。</span> <span><span class="t-mark-rev t-since-c11">(C11 起)</span></span></span>位域中的位数（ <span class="t-spar">宽度</span> ）限制其所能保有的值域：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
 <span class="co1">// 三位无符号位域，允许值为 0..7</span>
 <span class="kw4">unsigned</span> <span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">3</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">struct</span> S s <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="sy2">++</span>s.<span class="me1">b</span><span class="sy4">;</span> <span class="co1">// 无符号溢出</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d<span class="es1">\n</span>"</span>, s.<span class="me1">b</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 输出： 0</span>
<span class="br0">}</span></pre></div></div>
</div>
<p>允许将多个相邻位域打包在一起（通常如此）：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="co1">// 通常将占用 4 字节：</span>
    <span class="co1">// 5 位： b1 的值</span>
    <span class="co1">// 11 位：未使用</span>
    <span class="co1">// 6 位： b2 的值</span>
    <span class="co1">// 2 位： b3 的值</span>
    <span class="co1">// 8 位：未使用</span>
    <span class="kw4">unsigned</span> b1 <span class="sy4">:</span> <span class="nu0">5</span>, <span class="sy4">:</span> <span class="nu0">11</span>, b2 <span class="sy4">:</span> <span class="nu0">6</span>, b3 <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%zu<span class="es1">\n</span>"</span>,<span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 通常打印 4</span>
<span class="br0">}</span></pre></div></div>
</div>
<p>拥有零 <span class="t-spar">宽度</span> 的特殊<i>无名位域</i>打破填充：它指定下个位域在始于下个分配单元的起点：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="co1">// 通常将占用 8 字节</span>
    <span class="co1">// 5 位： b1 的值</span>
    <span class="co1">// 27 位：未使用</span>
    <span class="co1">// 6 位： b2 的值</span>
    <span class="co1">// 15 位： b3 的值</span>
    <span class="co1">// 11 位：未使用</span>
    <span class="kw4">unsigned</span> b1 <span class="sy4">:</span> <span class="nu0">5</span><span class="sy4">;</span>
    <span class="kw4">unsigned</span> <span class="sy4">:</span><span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// 开始新的 unsigned int</span>
    <span class="kw4">unsigned</span> b2 <span class="sy4">:</span> <span class="nu0">6</span><span class="sy4">;</span>
    <span class="kw4">unsigned</span> b3 <span class="sy4">:</span> <span class="nu0">15</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 通常打印 8</span>
<span class="br0">}</span></pre></div></div>
</div>
<p>因为位域不必在字节的起点开始，故不能取位域的地址。不可能有指向位域的指针。不能对位域使用 <a href="sizeof.html" title="c/language/sizeof"><code>sizeof</code></a> <span class="t-rev-inl t-since-c11"><span>和 <a href="_Alignas.html" title="c/language/ Alignas"><code>_Alignas</code></a> </span> <span><span class="t-mark-rev t-since-c11">(C11 起)</span></span></span>。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>位域的下列属性<i>未定义</i>：
</p>
<ul><li> 在位域上调用 <span class="t-lc"><a href="../types/offsetof.html" title="c/types/offsetof">offsetof</a></span> 的效果
</li></ul>
<p>位域的下列属性<i>未指定</i>：
</p>
<ul><li> 保有位域的分配单元的对齐
</li></ul>
<p>位域的下列属性为<i>实现定义</i>：
</p>
<ul><li> <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> 类型的位域被当做有符号或无符号
</li><li> 是否容许异于 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> 、 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">int</span></span></span> 、 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">int</span></span></span><span class="t-rev-inl t-since-c99"><span> 及 <span class="t-c"><span class="mw-geshi c source-c">_Bool</span></span></span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span> 类型的位域
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c11"><td>
<ul><li> 是否容许原子类型的位域
</li></ul>
</td>
<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td></tr>
</table>
<ul><li> 位域是否能越过分配单元边界
</li><li> 分配单元内位域的顺序（一些平台上位域从左往右打包，其他平台上是从右往左）
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<p>尽管 <code>_Bool</code> 的对象表示的位数至少为 <span class="t-lc"><a href="../types/limits.html" title="c/types/limits">CHAR_BIT</a></span> ， <code>_Bool</code> 类型位域的 <span class="t-spar">宽度</span> 不能大于 1 。
</p>
</td>
<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td></tr>
</table>
<p>C++ 编程语言中，位域的宽度能超出底层类型，并且 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> 类型的位域始终为有符号。
</p>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-17">
<ul><li> C17 标准（ISO/IEC 9899:2018）：
</li></ul>
<dl><dd><ul><li> 6.7.2.1 Structure and union specifiers 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-11">
<ul><li> C11 标准（ISO/IEC 9899:2011）：
</li></ul>
<dl><dd><ul><li> 6.7.2.1 Structure and union specifiers 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c99">
<ul><li> C99 标准（ISO/IEC 9899:1999）：
</li></ul>
<dl><dd><ul><li> 6.7.2.1 Structure and union specifiers 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c89">
<ul><li> C89/C90 标准（ISO/IEC 9899:1990）： 
</li></ul>
<dl><dd><ul><li> 3.5.2.1 Structure and union specifiers 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table>

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